The present invention relates to a half tone processing for use in image forming apparatuses such as a digital copying apparatus, particularly to an image processing apparatus for reproducing a half tone image by a screen processing.
An object of a screen processing is consider properties of recording apparatuses arranged in subsequent stages, modulate an input image signal to a mesh-dot image or multi-line form, and reproduce a half tone image. In the mesh-dot image form, a plurality of outputted dots form one mass, and the masses are regularly arranged like meshes of a net in the image.
In the screen processing of a multi-line system, a width of an output pixel is changed in accordance with a value of the input image signal and half tone is represented. There has heretofore been a technique of comparing a triangular wave or another reference signal outputted from an analog circuit with the input image signal and forming a multi-line screen. That is, a comparator compares a monotonously increasing or decreasing triangular wave with an image signal obtained by D/A conversion of input image data in a period of one or several pixels. For example, when an amplitude of the triangular wave is larger than that of the image signal, “H” is outputted (i.e., laser is turned ON), and the image is formed in the pixel. A period of the triangular wave is the same as that of the image signal. When the monotonously increasing triangular wave is used, the image is formed, for example, on a right side in the pixel. When the monotonously decreasing triangular wave is used, the image is formed, for example, on a left side in the pixel.
A type of an outputted image pattern is limited in the aforementioned conventional screen processing. That is, since a constitution is fixed, various screen patterns cannot be handled.